The invention relates to a paper machine wire for the wet end section of a paper machine, having a textile or non-textile fabric, particularly a woven fabric, the one side of which is directed to the paper and the other, opposite side of which is directed to the paper machine wherein the one side of the fabric, which side is directed to the paper, is at least partly covered by a fiber layer.
Such paper machine wires which may be manufactured in the form of a single or a multiple layer or in the form of a composite woven fabric and which generally comprise a woven or non-woven, textile or non-textile fabric, particularly a woven fabric, are known in practice. These fabrics serve for dewatering an aqueous fiber suspension and for guiding this suspension to the press section of the paper machine.
The known paper machine wires of all kinds of design are characterized by the fact that the side of the wire directed to the paper is determined by the structure given by the weave of the woven fabric. The surface structure of the woven fabric shall indicate on the paper as few marking as possible. Thus, there is a particular demand for a reduced topographical marking of the wire due to the sensitive paper types. With the known paper machines the retaining capability, also called retention, required by the paper manufacturer is often not sufficient with respect to fibers as well as auxiliary and additional material. Moreover, a washing out may occur starting from the running or wearing side of the woven fabric caused by the dewatering and guide elements of the paper machine, said washing out also leading to low retention values.
A paper machine wire, also called paper machine forming fabric, according to the preamble of claim 1, comprising a textile fabric having the form of a woven fabric is known from U.S. Pat. No. 5,077,116. The fiber layer, also called fiber batt, is formed by individual fibers and is loosely put on the surface of the woven fabric and connected with the threads of the woven fabric forming the surface of the woven fabric. It is shown particularly in FIGS. 5, 8, 11 and 14 that the fiber layer is adapted to the surface of the woven fabric such that the structure of the woven fabric is imitated. As a result, the surface of the fiber layer similar like the surface of the woven fabric does not form a uniform horizontal plane.
It is therefore an object of the present invention to provide a paper machine wire of the initially specified kind according to which the topographical marking tendency is reduced to a minimum.
It is a further object of the present invention to provide a paper machine wire, the retention values of which are amended regarding the paper production.
These and other objects are accomplished according to the present invention by providing a paper machine wire comprising a fiber layer which is permanently compressed in some regions less intensively than in other regions. According to the present invention the fiber layer is less intensively compressed in those regions where corresponding regions of the surface of the side of the textile or non-textile fabric directed to the paper lies in a plane mainly forming the surface of the fabric. Therefore, the fiber layer has a complete uniform, smooth surface. Moreover, the permeability of the fiber may be adjusted by the orientation of the fibers and the structure of the fiber layer. As a result, the paper manufactured by such a paper machine wire is practically free from any marking and therefore satisfies the highest requirements. As a further result the retaining capability with respect to the fibers and the auxiliary as well as additional materials of a fiber suspension may extensively be adjusted depending on the density or the pore size of the selected fiber. This is also true regarding the ability of the fiber layer to dewater the fiber suspension.
Preferred embodiments of the invention are mentioned in the subclaims.
According to an advantageous further embodiment the fiber layer has a thickness of preferably 0.1 to 1 mm as well as a substantially uniform permeability. The mentioned thickness of the fiber layer increases the total thickness of the paper machine wire by a delimited extent. A uniform permeability of the fiber layer further enables a uniform dewatering of the fiber suspension and, therefore, a substantially homogeneous consistency of the fibers guided to the wet end section, also simply called wet end, of the paper machine.
According to another embodiment of the present invention the fiber layer is made of plastic, fusible fabrics, such as for example polyethylene terephthalate, polyamide, polyethylene, polypropylene, polybutylene terephthalate and combinations of the aforementioned materials. Those plastic fibers enable, as discussed hereafter, a thermal treatment and, therefore, an easy depositing of the fiber layer on the upper side of the textile or non-textile fabric.
It is further advantageous that the surface of the fiber layer is permanently topographically structured preferably by circular, triangular, rectangular depressions or by depressions formed in another way or that the surface of the fiber layer is produced with hydrophilic, hydrophobic or anti-static or stain releasing properties by means of physical or chemical surface treatment methods. The selected topographies may be realized according to the desire of the paper manufacturer by a simple configuration of the surface of the fiber layer on the paper independently from the structure of the fabric. It is therefore possible to provide predetermined patterns on the paper by correspondingly forming the fiber layer independently from the structure of the fabric. Thus, the structure of the paper may be configured in at least any manner.
According to a further embodiment of the present invention the fiber layer is penetrated by pores which may taper or widen toward the fabric. Particularly the retaining capability of the total paper machine wire as well as the three dimensional dewatering characteristic may be adjusted as desired by the pore size or a pore size range, and the retaining capability and the dewatering characteristic may be adapted to the size of the fibers which are to be retained.
It is further advantageous that the textile or non-textile fabric has on its paper directed side fibers or threads, also called yarns, which have the capability of being permanently attached to the fiber layer, wherein the fibers or threads are preferably made of plastic fusible fibers, such as polyethylene terephthalate, polyamide, polyethylene, polypropylene, polybutylene terephthalate and combinations of the aforementioned materials. As a result, fibers or threads of the fabric and the material of which the fiber layer is made, may at least partly be identical and, therefore, may easily permanently attached to each other.
According to a further embodiment of the present invention the fiber layer is fixed on the fabric by a bonding agent preferably a hot melt glue. The fiber layer may particularly perform a permanent bonding with the fabric by means of a heat-pressure-treatment, wherein the temperature is between about 170 and 270xc2x0 C. and the line pressure is between about 10 and 50 N/mm depending on the selected materials. According to a another embodiment of the present invention the fiber layer is permanently fixed on the textile fabric by ultrasonics, the welding frequency being between 15 and 72 MHz, preferably between 15 and 35 MHz. The advantage of the above mentioned method lies in the fact that the materials partly dissolve in each other such that a close bonding between the fiber layer and the textile or non-textile fabric is performed. The advantage of the above mentioned ultrasonic method mainly lies in the fact that only the direct welding joint is heated whereas the surroundings of the welding joint remain cold. The fiber layer may purposefully be deposited on the surface of the fabric thereby and may be attached thereto. Non-desired thermal deformations or thermal decompositions due to a too high temperature occur less often and are restricted to certain regions.
According to still a further embodiment the covering ratio between the fiber layer and the side of the fabric directed to the paper is more than 40%. This enables a slower and, therefore, more carefull dewatering of the fiber suspension compared to a less covered surface of the textile or non-textile fabric.